I have found a way to get rid of the buzzing sound coming from the iMac CPU fan. I have a fan in my PC that does the same thing, and to fix it I do something similar to what I did to my iMac. Reboot, hold down the alt key (or option key) and allow it to go into option boot mode. All the fans will whizz up to full speed. Leave them running at full speed for an hour or two. After this period, boot up ino Mac OS X. The buzz has gone and it's now a fffffffff sound, which is far less annoying. In my PC I just turn the fan up to max speed for an hour or two, it stops it making buzz or click sounds for a few weeks.